This invention relates to improved wires or cables of aluminum or alloys of aluminum and particularly to processes for treating aluminum wire with a pickling bath and thereafter plating the treated wire with nickel.
Wires and cables used as electrical conductors have been primarily made of copper. In recent years for economic reasons aluminum has been used in place of the more expensive copper. However, while aluminum is cheaper than copper, aluminum has certain disadvantages such as inferior mechanical properties and lower conductivity. Another serious disadvantage of aluminum and alloys thereof is the difficulty to make good contacts with themselves or other metals. For example, an aluminum wire cannot be readily soldered to another aluminum wire or to another metal. This makes it difficult and very dangerous to use aluminum wires or cables for instance in house wiring. When the term "aluminum" is used hereinafter, it shall refer to both the aluminum metal and alloys of aluminum.
To obtain aluminum wire free of the aforeindicated disadvantages, aluminum wire has been coated with other metals such as copper to obtain aluminum-metal bonds. These coated wires have better conductivity and make better contacts. Such coatings can be obtained by heat-treatment of the aluminum with the metal copper as described in Texas Instruments Bulletin No. 516-TB 25-468 or through electrochemical deposition of the metal on the aluminum. However, the bonds between the coating and the aluminum were not strong and on use the coating frequently rubbed off or peeled off. Furthermore, the process was not readily controllable to provide a coating of both uniform and desired thickness. Usually, the coatings were thicker than desired, thereby increasing the costs and reducing the ductility of the coated wires. Desirably, the thickness of the coating should be about 1.0 to 1.5 microns.
These deficiencies were overcome in U.S. Pat. No. 3,867,265 wherein an aluminum wire, of either pure aluminum or an aluminum alloy, is plated with a firm layer of at least one of nickel, copper, tin, zinc or cadmium by treating the wire with an aqueous solution of hydrofluoric and hydrochloric acids and then electroplating the wire with the metal. The electrolyte is a solution of a fluoroborate or sulfamate of the metal. The aluminum wire so plated possesses superior conductivity and may be readily soldered.
However, it has been found that the preferred plating material is nickel. It has also been found that the pickling step of the process creates several problems. Aluminum bodies are difficult to pickle so that the requisite degree of surface purity for a subsequent electroplating will be obtained. This depends primarily on the preventive oxide film which covers the aluminum. It is certainly not especially thick, but it is still a problem partly because of the speed with which it reforms. A special problem is the silicon which exists in aluminum, as an alloying constituent in some cases or as impurity in other cases. After pickling in conventional baths, which are either acid or basic, there often remain small areas of silicon containing films. To remove these silicon containing films one must resort to strong acid-baths which contain hydrofluoric acid as in the above-mentioned U.S. Patent. Such pickling baths have however the disadvantage that a very uneven and unnecessarily strong etching of not-silicon-containing surface areas is obtained. Another disadvantage is that hydrofluoric acid is very poisonous and it easily causes a state of illness for those who work with it, thus requiring that strict measures of precaution be taken.